Light-emitting device having a phosphor particle layer with specific thickness

ABSTRACT

A light emitting device having a die that includes a light source that generates light of a first wavelength and a layer of phosphor particles covering the die is disclosed. The phosphor particles convert a portion of the light of the first wavelength to light of a second wavelength. The light source can be fabricated by attaching the light source to a substrate, and converting the light source by applying a light converting layer that includes a volatile carrier material and particles of a phosphor that convert light of the first wavelength to light of the second wavelength over the light source. The volatile carrier material is then caused to evaporate leaving a layer of the phosphor particles over the light source. A binder material can be incorporated in the volatile carrier for binding the phosphor particles to one another after the volatile carrier material is evaporated.

FIELD OF THE INVENTION

The present invention relates to light emitting diodes(LEDs) thatutilize phosphors to convert a portion of the light generated by theLED.

BACKGROUND OF THE INVENTION

For the purposes of the present discussion, the present invention willbe discussed in terms of a “white” emitting light-emitting diode (LED);however, the methods taught in the present invention can be applied towide range of LEDs. A white emitting LED that emits light that isperceived by a human observer to be “white” can be constructed by makingan LED that emits a combination of blue and yellow light in the properratio of intensities. High intensity blue-emitting LEDs are known to theart. Yellow light can be generated from the blue light by convertingsome of the blue photons via an appropriate phosphor. In one design, atransparent layer containing dispersed particles of the phosphor coversan LED chip. The phosphor particles are dispersed in a potting materialthat surrounds the light-emitting surfaces of the blue LED. To obtain awhite emitting LED, the thickness and uniformity of the dispersedphosphor particles must be tightly controlled.

In one prior art method for constructing such a device, the phosphor ismixed with a resin material such as epoxy or silicone and the slurry isput over the LED chip. The phosphors are typically in the form of fineparticles and usually have a distribution typically ranging from 1 um to20 um. When the slurry is used to cover the LED chip, the phosphorparticles are initially distributed throughout the coating layer andoccupy a volume greater than the LED chip.

Such devices have a number of problems. First, if the resin does notcure quickly, the phosphor particles tend to settle, and hence, there isa non-uniform distribution of particles that often has a boundarybetween the region of the resin having the particles and the upperportions of the resin coating. This boundary can cause the coating tosplit into two layers at some point in the life of the light source.

Even if the resin sets before the particles have time to settle, theresulting light source is a three-dimensional source of a size that ismuch larger than the underlying LED chip. Such a source presentsproblems in applications in which an optical system must be used toimage the light source onto an object that is to be illuminated. Thelight source is essentially a compound light source having a first pointsource that emits blue light and a broader diffuse source that emits theyellow light. Consider an optical system that images this compoundsource onto a scene that is to be illuminated with white light. To beperceived as white light, each area of the scene must receive the sameamount of blue and yellow light. Consider a collimating lens that hasthe LED at its focal point. The blue light will be formed into a beamhaving a more or less uniform intensity. The yellow light will, ingeneral, not be uniformly distributed across this beam, since the yellowlight source is not at the focal point of the lens and consists of abroad three-dimensional source. Hence, a human observer will see asource that varies in color across the source.

SUMMARY OF THE INVENTION

The present invention includes a light emitting device having a lightsource that generates light of a first wavelength and a layer ofphosphor particles covering the die. The phosphor particles convert atleast a portion of the light of the first wavelength to light of asecond wavelength. The layer of phosphor particles preferably has athickness of less than 100 μm, and may include the residue of a slurryof the phosphor particles and a volatile solvent. The residue is theportion of the slurry that remains after the volatile solvent is drivenoff. In one embodiment, the light source is an LED. In one embodiment, alayer of clear encapsulating material covers the layer of phosphorparticles. The encapsulating material can include a diffusing materialfor scattering light generated by the light source and the phosphorparticles. In one embodiment, the die is located in the cup such that aportion of the light generated by the light source is reflected from thecup. The layer of phosphor particles covers a portion of the cup in thisembodiment. In one embodiment, a clear layer is placed between the lightsource and the phosphor layer.

A light source according to the present invention can be fabricated byattaching the die having the light source to a substrate. A lightconverting layer that includes a volatile carrier material and particlesof a phosphor that convert light of the first wavelength to light of thesecond wavelength is applied over the die. The volatile carrier materialis then caused to evaporate thereby leaving a layer of the phosphorparticles over the die. The light converting layer can also include abinder material for binding the phosphor particles to one another whenthe volatile carrier material is evaporated. In one embodiment, thecarrier material includes epoxy resins, silicone, polyurethanes,polyvinyl acetates, cyanoacrylate, phathalate, glass, aluminum nitrideor silicone dioxide. In one embodiment, the binder material includes apolymer that includes acrylic resin, di-butyl phathalate, diacetonealcohol, or tetra ethyl orthosilicate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art LED light source thatutilizes phosphor conversion.

FIG. 2 illustrates a light-emitting device according to one embodimentof the invention.

FIG. 3 is a cross-sectional view of another embodiment of a light sourceaccording to the present invention.

FIG. 4 is a cross-sectional view of another embodiment of a light sourceaccording to the present invention.

FIG. 5 is a cross-sectional view of another embodiment of a light sourceaccording to the present invention having a clear layer between thelight source and the phosphor layer.

FIG. 6 is a cross-sectional view of another embodiment of a light sourceaccording to the present invention having a diffusing layer above thephosphor layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The manner in which the present invention provides its advantages can bemore easily understood with reference to FIG. 1, which is across-sectional view of a prior art LED light source that utilizesphosphor conversion. Light source 100 has an LED 105 mounted in a cavity110 on the first terminal 115 of a substrate using an adhesive 120. Anelectrical connection 125 is made from one end of the LED to anotherterminal 130 of the substrate. A layer of coating is dispensed insidethe cavity to cover the LED. The coating layer includes a mixture ofphosphor 140 in an epoxy material 145. A clear epoxy 150 encapsulatesthe whole assembly forming an interface between it and the coatinglayer. Each phosphor particle acts as a point light source for light atthe converted wavelength. Light that is not converted either escapes thephosphor layer without scattering, and hence, originates at a sourcebelow the phosphor particles or is scattered. The scattered lightappears to originate from a more diffuse source of blue light.

The clear encapsulating plastic structure 150 can act as a lens and/or alens is placed above the light source. In either case, the lens ispresented with a number of different light sources at a variety ofdepths in the epoxy layer. Since the light color changes with depth,providing a uniform color at all points that are illuminated by theoptical system is difficult.

The present invention overcomes this problem by confining the phosphorparticles to a thin layer over the LED chip. In the proposed invention,a wavelength converting material such as phosphor is laid over the LEDchip in a manner such that substantially all the phosphor particles arein contact with the LED chip and the walls of the cavity where the LEDis mounted.

Refer now to FIG. 2, which illustrates a light-emitting device accordingto one embodiment of the invention. Device 400 is analogous to device100 shown in FIG. 1. Device 400 has an LED 405 mounted in a cavity 410on the first terminal 415 of a substrate using an adhesive layer 420. Anelectrical connection 425 is made from one end of the LED to anotherterminal 430 of the substrate. A layer of phosphor 435 covers the LEDand at least part of the walls of cavity 410. An encapsulate material440 encapsulates the LED, phosphor layer, and part of the terminals. Theencapsulate layer 440 is configured to provide a lens 441 in thisembodiment of the present invention.

However, it should be noted that the lens function is optional. Anembodiment without a lens is shown in FIG. 3, which is a cross-sectionalview of another embodiment of a light source according to the presentinvention. Device 500 has an LED 505 mounted in a cavity 510 on a firstterminal 515 of a substrate using an adhesive 520. An electricalconnection 525 is made from one end of the LED to a second terminal 530of the substrate. A layer of phosphor 535 covers the LED and partiallycovers the walls of the said cavity. An encapsulant material 540encapsulates the LED covered with said phosphor and fills the cavity.The walls of cavity 510 preferably reflect the light generated by theLED and the phosphor, and hence, increase the light output as well asproviding a collimating function.

While the above embodiments utilize some form of reflecting well toimprove the light efficiency of the light source, embodiments of thepresent invention that lack such a cavity can also be constructed. Refernow to FIG. 4, which is a cross-sectional view of another embodiment ofa light source according to the present invention. Device 600 has an LED605 mounted on a first terminal 610 of a substrate using an adhesive615. An electrical connection 620 is made from one end of the LED to asecond terminal 625 of the substrate. A layer of phosphor 630 covers theLED and a portion of the substrate. An encapsulant 635 encapsulates theLED covered with the phosphor on one side of the device.

The present invention utilizes a much thinner phosphor layer than priordevices. Therefore, the size of the light source comprising both the LEDchip and the phosphor particles is smaller. This feature is useful whensecondary optical systems are utilized to collimate or image the lightfrom the light source.

In addition, the present invention can be used to create phosphor layershaving multiple sub-layers. Each sub-layer can be deposited in themanner described above. The individual sub-layers can be formed fromphosphor particles having different particle sizes and/or compositions.For example, the second sub-layer can be constructed from finer phosphorparticles to provide more uniform coverage. In another example, thedifferent sub-layers include phosphors that provide differentwavelengths in the final output light generated by the light source.

In addition, a light source according to the present invention requiresonly a single layer of encapsulant. Hence, there is no interface ofdissimilar material between the phosphor layer and the bulk encapsulantlayer that can lead to delamination of the layers at the interface asdiscussed above.

Having explained the structure of a light source according to thepresent invention, the manner in which the light source is constructedwill now be described in more detail. The present invention isconstructed by covering the LED chip with a mixture of phosphorparticles in a carrier material that can be treated to drive away aportion of the carrier material, and thus, leave a layer of phosphorparticles behind. The phosphor particles are uniformly distributed in alayer of carrier material that is dispensed over the LED chip. The layeris baked to at least partially drive away the carrier material leavingbehind a layer of phosphor. Consequently, the phosphor particles allsettle down and are deposited around the LED chip and at least someportions of the cavity walls.

The carrier material preferably has a tacky texture so that the phosphorparticles can adhere to one another and to the LED chip and cavitysurfaces. Further, the carrier material preferably can be at leastpartially dried by baking at a temperature not more than 300 degrees C.,more preferably at a temperature not more than 200 degrees C.Additionally, it is preferable that any residue left behind when thecarrier material is driven off be transparent to visible light.

The carrier material can be an organic material or a polymer such asepoxy resins, silicone, polyurethanes, polyvinyl acetates, cyanoacrylateand phathalate. The carrier material can also be an inorganic materialsuch aluminum nitride and silicone dioxide. For example, in oneembodiment of the present invention, phosphor particles are mixed withsilicone dioxide in a volatile solvent and the mixture dispensed or spunaround the LED chip. The coated LED is then baked to drive off thevolatiles leaving a layer of phosphor around the LED chip.

As noted above, the residue of the carrier material preferably also actsas an adhesive to bind the phosphor particles to one another and to theLED and surrounding surface. To provide this functionality, a bindermaterial can be added to the carrier material. For example, one or morepolymers can be added to the carrier material. Binder polymerscomprising acrylic resin, di-butyl phathalate, diacetone alcohol, andtetra ethyl orthosilicate can be utilized for this purpose.

The above-described embodiments of the present invention utilize aphosphor layer that is formed on the LED chip and surrounding area.However, other configurations can also be utilized. For example, atransparent layer can be introduced between the LED chip and thephosphor layer. An embodiment of the present invention having such alayer is shown in FIG. 5. Light source 700 includes a transparent layer710 that covers chip 720 and separates chip 720 from phosphor layer 730.Such a transparent layer can be used to prevent physical contact betweenthe chip and the phosphor layer. In addition, the material used can bechosen to shield the phosphor layer from UV or heat generated in thechip.

If a diffuse extended light source is required, a diffusing compound canbe introduced into an epoxy layer that is placed over the phosphor layerprior to encapsulating the device in a clear bulk epoxy layer. Anembodiment having such a diffusing layer is shown in FIG. 6. Referringto FIG. 6, light source 750 includes a chip 720 that is covered by aphosphor layer 760. A diffusion layer 761 is deposited on top ofphosphor layer 760 prior to encapsulation by the bulk epoxy layer 770.Diffusion layer 761 can be constructed from a clear epoxy that hasscattering particles dispersed therein.

The above-described embodiments of the present invention utilize an LEDfor the light source. However, embodiments of the present invention thatutilize other light sources can also be constructed. For example, alight source based on a semiconductor laser could also be utilized.

To simplify the drawings and better explain the present invention, theabove figures show layers of relatively large phosphor particles thatare only a few particles thick. However, it is to be understood that thephosphor layers are actually uniform layers constructed from muchsmaller particles, and the layers are many particles thick.

Various modifications to the present invention will become apparent tothose skilled in the art from the foregoing description and accompanyingdrawings. Accordingly, the present invention is to be limited solely bythe scope of the following claims.

1. A light emitting device comprising: a light source that generateslight of a first wavelength, said light source comprising a die thatemits light at said first wavelength; a conformal layer of phosphorparticles covering said die and in direct contact therewith, saidphosphor particles converting at least a portion of said light of saidfirst wavelength to light of a second wavelength, said layer having asubstantially uniform thickness of less than 100 μm over said die; and areflecting cup comprising reflective sidewalls, said die being locatedin said cup such that a portion of said light generated by said die isreflected from said cup, said layer of phosphor particles covering aportion of said reflective sidewalls.
 2. A light emitting devicecomprising: a light source that generates light of a first wavelength;and a layer of phosphor particles covering said light source, saidphosphor particles converting at least a portion of said light of saidfirst wavelength to light of a second wavelength, said layer comprisinga residue of a slurry of said phosphor particles in a volatile solvent,said residue comprising the portion of said slurry that remains when aportion of said volatile solvent is removed.
 3. The light emittingdevice of claim 2 wherein said light source comprises an LED.
 4. Thelight emitting device of claim 2 further comprising a layer of clearmaterial covering said layer of phosphor particles.
 5. The lightemitting device of claim 4 wherein said clear material further comprisesa diffusing material for scattering light generated by said light sourceand said phosphor particles.
 6. The light emitting device of claim 5further comprising a layer of transparent bulk encapsulating materialover said layer of clear material.
 7. The light emitting device of claim2 further comprising a reflecting cup comprising reflective sidewalls, adie located in said cup such that a portion of said light generated bysaid die is reflected from said cup, said layer of phosphor particlescovering a portion of said reflective sidewalls.
 8. The light emittingdevice of claim 2 further comprising a layer of clear material betweensaid light source and said layer of phosphor particles.